1. Field of the Invention
The invention relates to a method and an apparatus for high-pressure end exhaust gas recirculation on an internal combustion engine combined with a turbocharger.
2. Discussion of Background
Exhaust gas recirculation at the high-pressure end of a supercharged internal combustion engine with a negative scavenging drop, i.e. with a small charge air pressure relative to a higher exhaust gas backpressure after the cylinder, is realized by simply connecting the exhaust gas duct system to the charge air duct system. In the case of a positive scavenging drop, however, such a connection is substantially more complicated because the pressure drop in the reverse direction has to be overcome.
An internal combustion engine with an exhaust gas turbocharger which is known from DE-C1 42 09 469 permits the exhaust gas recirculation in the case of a negative scavenging drop and, more particularly, in the case of a positive scavenging drop also. In this solution, the turbine of the exhaust gas turbocharger is configured in such a way that it compresses some of the exhaust gases of the internal combustion engine in order to overcome the pressure drop relative to the charge air pressure. The compressed exhaust gases are introduced into the inlet duct of the internal combustion engine by means of an exhaust gas recirculation duct between the charge air cooler and the compressor of the exhaust gas turbocharger.
This solution, however, requires a complicated and specially configured turbine with an integrated exhaust gas compressor part. It is therefore impossible to employ commercial exhaust gas turbochargers. Because the compression energy necessary increases with the induction temperature and an exhaust gas compressor part integrated into the turbine cannot be designed in an optimum manner, a very high expenditure of energy is required for such compression. This reduces the attainable efficiency of the exhaust gas recirculation.
A supercharged internal combustion engine is known from DE-A1 43 20 045 in which the disadvantages mentioned above are substantially obviated. For this purpose, the exhaust gases are subdivided into a plurality of exhaust gas flows. The first exhaust gas flow is fed into the exhaust gas turbine of the exhaust gas turbocharger. A second exhaust gas flow is led back to the internal combustion engine in a separate exhaust gas duct. In this arrangement, the second exhaust gas flow is initially cooled before entering the internal combustion engine and is subsequently recompressed. In order to limit the charge pressure, a third exhaust gas flow is led away directly to the exhaust gas system via a by-pass duct.
The consequence of removal without use of some of the exhaust gases is, however, an energy loss which lowers the possible efficiency of the exhaust gas recirculation. Another disadvantage of this solution is that the compressor for the second exhaust gas flow is driven by the internal combustion engine. This lowers the efficiency of the internal combustion engine. The drive for the compressor takes place by means of a gear which is separately connected to the crankshaft of the internal combustion engine. This gear substantially increases the technical outlay for the exhaust gas recirculation and, therefore, the costs as well. The outlay is additionally increased by the multiplicity of control valves and the control system necessary for them. In addition, the mechanical exhaust gas compressor coupled to the crankshaft supplies an exhaust gas quantity which is not, generally speaking, proportional to the air quantity induced by the internal combustion engine. In order to realize a constant exhaust gas recirculation rate even in the case of changing loads, a strongly correcting and therefore complicated control system is necessary.